A gated oscillator is responsive to an enabling signal to selectively condition the oscillator to start or stop oscillating.
In a known type of gated oscillator, periodic pulses are produced by charging and discharging a capacitor between first and second voltage levels. The capacitor is periodically charged from a first voltage level to a second voltage level, and then discharged from the second voltage level to the first voltage level. The potential on the capacitor therefore oscillates between the first and second voltage levels.
The oscillator is stopped by preventing the periodic charging and discharging of the capacitor. When the oscillator is stopped, the capacitor continues to discharge past the first voltage level towards the lower power supply potential. When the oscillator is started again, the capacitor begins charging from a lower potential than the first voltage level so that the first period of oscillation is longer than the steady state period of oscillation.
Therefore, a gated oscillator of the type described above, exhibits an error in the width of the first pulse. In certain applications, such first pulse error is undesirable. Specifically, in an automotive application, a gated oscillator is used as part of a system for converting analog quantities to digital form. Physical engine parameters (e.g. temperature, pressure, etc.) are sensed by the impedance change of respective sensors. The impedance change is converted in a charging circuit into a pulse width modulated signal.
The pulse width modulated signal operates the gated oscillator by starting the oscillator at the leading edge of the pulse and stopping the oscillator at the trailing edge thereof. The resulting output pulses from the gated oscillator are accumulated in a digital counter. The contents of the digital counter represent a digital measurement of pulse width, which is therefore proportionally related to the digital value of the sensed analog parameter. The first pulse error of the gated oscillator described above therefore causes an error in the digital measurement of the physical parameter.
The present invention is directed to a gated oscillator wherein the first pulse error is substantially eliminated.